1. Field of the Invention
This invention relates to aircraft on-board gas generating apparatus.
2. Description of the Prior Art
Oxygen-enriched air for breathing by aircrew is now frequently produced by an on-board oxygen generating system (OBOGS) in which compressed air bled from an engine of an aircraft is fed to molecular sieve beds containing molecular sieve material having an affinity for retention of nitrogen. The molecular sieve beds are cycled so as to place the beds in overlapping charge/adsorption on-stream and purge/desorption regeneration phases whereby oxygen-enriched air is continuously supplied as product gas.
EP-A No. 0,129,304 (Normalair-Garrett) discloses an OBOGS having three molecular sieve beds and a fixed logic sequencer unit which provides two different overall cycle times for the beds and fixes the relative duration of each phase within the overall cycle time. A switching device which includes a fleuric partial pressure sensor more particularly disclosed in EP-A No. 0,036,285 (Normalair-Garrett), responds to the pertaining concentration of a constituent gas in the product gas delivered by the system and switches between two overall cycle times so as to maintain the concentration of oxygen in the oxygen-enriched air supplied as product gas within predetermined limits by varying the saturation level of the beds and consequently their sieving capability.
Space within an aircraft fuselage is at a premium so that the OBOGS should require the smallest possible space envelope. In minimising the space envelope required to house the OBOGS we have proposed a molecular sieve bed container having a plurality of molecular sieve bed chambers formed by tubular wall members concentrically located within a tubular outer housing and closed at their ends by end closure members. This container is disclosed in EP-A No. 0225736 (Normalair-Garrett). One end closure member of the container incorporates valves for controlling supply of charge air to the sieve bed chambers and venting purge gas from the sieve bed chambers whilst the other end closure member incorporates valves for controlling passage of product gas from the sieve bed chambers to an outlet and portways for permitting some of the product gas to flow back through the sieve bed chambers as purge gas. In a preferred embodiment the valves comprise diaphragm valves, opening and closing of the valves for supply of charge air to the sieve bed chambers and venting of purge gas from the sieve bed chambers being controlled by sequentially operating pneumatic servo-valves switched by an electronic timing device. According to the number of sieve beds and the molecular sieve material with which they are filled the container may be adapted to provide either or both of supplies of oxygen-enriched and nitrogen-enriched product gases. This arrangement provides within a single container a plurality of molecular sieve beds and as such is particularly suited for installation in an aircraft having a limited space envelope for housing an on-board gas generating system.
However, with increasing demands by other aircraft systems for supplies of engine compressor stage bleed air, there is a requirement for a compact on-board gas generating apparatus which does not require bleed air from the engine as its supply gas. Also, it would be advantageous if such apparatus had a minimal requirement for use of electrical systems in control of its operation.